Since the PS3's error 8011... Whatever the hell it is, is ruining more PS3's than originally expected, I wouldn't call it invincible any more. Less common than rrod, but still common enough to be an issue.
Halo 3 fell far short of its prequels as well as all expectations about it in my opinion. Gears of War is a good series. Overall the only exclusive 360 games I would say are outstanding would be Halo 1&2 and the Gears of War series.
The PS3 is pretty solid. You won't be sold a PS3 without a hardrive at least ;p Online is just tolerable, but it's free and I certainly wouldn't like to pay for online gaming in addition to internet, electric, etc. A couple good PS3 exclusives are Disgaea 3, Killzone, the Final Fantasy series all the way back to 7, SOCOM (yes the newest one is now stable), Resistance 1&2, and the Metal Gear series.
That said, you can buy a naked 360 for about $200, or a stripped-down PS3 for about $350. I would say try to get one of the old 80-gig PS3s, which would probably be about $450.